Which of the following words is closest in meaning to “Impregnable”?

Impregnable means too strong to be captured or defeated.

• Correct answer - unconquerable: it has the same meaning as Impregnable.
• unconvincing: not persuasive - wrong, it does not mean the same as Impregnable.
• weak: lacking strength - wrong, it does not mean the same as Impregnable.
• vulnerable: open to attack or harm - wrong, it does not mean the same as Impregnable.